Values of bulk osmotic potential at full turgor (ψπo), water potential at which the bulk turgor pressure first reaches zero (ψz) and maximum bulk elastic modulus (ε max ) were monitored monthly for 13 months in the shoot and root systems of 2- to 3-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings using the pressure-volume method. In shoots, ψπo and ψz went through two maxima and minima throughout the year; the minima for ψπo and ψz were - 2.3 and - 3.8 MPa, respectively, in midwinter and - 2.5 and - 3.5 MPa, respectively, in midsummer. ε max was lowest (elasticity highest) in April (5.0 MPa) and highest in September (17.0 MPa). In root systems ψπo was nearly constant at - 1.0 MPa all year, while ψz dropped to a minimum annual value of -3.0 MPa in July and reached maxima (-2.0 MPa) in March and September. Turgor maintenance capacity changed seasonally due to changes in ψπo (osmotic adjustment). Dynamics of these and other tissue water relations properties are discussed in relation to phenological developmental stages, frost hardiness, and drought tolerance. Forest Sci. 30:538-548.
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Ritchie et al. (1984) studied this question.